A closure device for a fluid system for medical purposes is disclosed. The closure device includes a support housing, which has at least one opening to a fluid reservoir or a fluid line, and also with a connection piece, which includes a Luer connecting portion for connecting to a syringe or some other Luer connection part, and with an elastically deformable closure membrane, which closes a fluid passage of the connection piece and opens it when the syringe or other Luer connection part is connected. A nonreturn valve is configured for opening both in the direction of the connection piece and in the direction of the opening to the fluid reservoir or to the fluid line is arranged in the support housing at a distance from the closure membrane in the direction of the fluid. The closure device has applications in several areas, including in infusion systems for medical purposes.
